NO145232B - POWDER EXPANSION FOR CEMENT, PROCEDURE FOR PREPARING IT AND USING THEREOF - Google Patents

Description

Mixtures containing hydratables

cements (usually Portland cement) tend to shrink when drying. It is recognized to counteract this undesirable tendency by incorporating into the mixes an expansion additive which expands at the correct stage during the drying of the mix (see e.g. Gen . Civ. 109, 285 (1936 H. Lossier), U.C. SESM Report No. 72-13 (1973, G. Komendant et al.), Plan. Bau 2, 351 (1951

H. Bickenbach), Concrete Technology and Practice, 3rd ed., 359

(1969, W.H. Taylor) and US-PS Nos. 3,519,449, 3,649,317, 3,801,339,

3,883,361, 3,884,710, 3,947,288 and 4,002,483). In a number of the known expansion additives, the active ingredient is

part of free lime (CaO) which is also known as unslaked lime, which expands by approx. 100% of its volume when hydrated,

the other ingredients being present to prevent the hydration from taking place before the desired stage in the drying of the cement mixture. Free lime is one of the constituents of Portland

cement, but an excess of this is usually considered undesirable,

and an addition of free lime to cement mixtures is avoided. Free lime does not serve as an expansion additive because it hydrates very quickly and therefore expands far too early during the binding process for it to be effective. It is also known (see, for example, US-PS No. 3,106,453 and German Patent Document No. 1,216,753) to subject free lime to certain special treatments with water and/or CO 2 to produce a lime which hydrates with slower rate, but the products obtained are ineffective as expansion additives. Likewise are the products obtained by exposing lime to the atmosphere, which contain Ca(0H)2

and CaCO-j, ineffective as expansion additives. It has also been proposed in US-PS No. 1,732,409 to produce quick-

binding lime products that contain a relatively high proportion of lime, with at least part of the lime being in the form of a product that has been obtained by heating finely divided lime in an atmosphere containing CC>2 at a temperature of 500 - 850°C

until the increase in weight has become 3 - 40% based on the weight of the starting material.

It has now been found that excellent expansion additives for cement mixtures can be prepared by heating particles comprising CaO in the presence of water vapor and/or CO^, or by simply heating particles comprising

CaO and a sufficient amount of calcium hydroxide which has been formed by absorption of water vapour.

According to the invention, a powder-

shaped material which is usable as an expansion additive for hydratable cement mixtures and which comprises calcium oxide and other ingredients which delay hydration of the calcium oxide#and the powdery material is characterized by the fact that it essentially consists of particles with a size below 250 µm and comprising a core of calcium oxide and around the core a protective coating of calcium hydroxide and/or calcium carbonate, and that it contains 36 - 95% by weight calcium oxide and has an expansion factor of at least 0.06%.

Such mixtures can be produced by a method according to the invention, which is characterized by the fact that particles which (a) have a size of less than 250 µm and (b) comprise at least 36% by weight of CaO, are heated, the particles being kept at a temperature of at least 100° C and, if the particles originally contain less than 8% by weight of Ca(OH)2, the particles are kept at a temperature of 100-400°C in an atmosphere comprising water vapour, or at a temperature of 350-850°C in an atmosphere comprising carbon dioxide, the heating being carried out under such conditions that the product will contain 36-95% by weight CaO and have an expansion factor of at least 0.06%.

The invention also relates to the use of the present calcareous powdery material in a cement mixture comprising a Portland cement, water and the powdery material in an amount of less than 30%, preferably 3-10%, based on the total weight of the cement and the powdery material.

The calcareous powdery material according to the invention is also very well suited for use in the production of a shaped object in which a cement mixture as defined above is cast.

Unless otherwise stated, parts and percentages used herein are based on weight. The expansion factor for a calcareous mixture is defined herein as the percentage increase in length of a test prism (test rod) that has been cured in water for 7 days after casting, according to ASTM C490-70 and C157-69T, from a mortar prepared according to ASTM C305- 65 and containing 0.1 part of the calcareous mixture, 0.9 part Portland cement of type ill, 2 parts washed sand No. 20 and 0.4 part water. The expansion factor for the treated lime qualities according to the invention is at least 0.06%, as a rule 0.07 - 2.5%, and preferably 0.07 - 0.4%. The expansion factor is a measure of the protective coating's effect in delaying the access of water to the core of calcium oxide and i.a. also for the calcium oxide content in the mixture.

The larger the particle size of the treated lime, the higher the lime content for a given thickness of the protective coating. For this reason, the particle size is generally at least l^um. If, on the other hand, particles with a size above 250 µm are used, they will cause an undesirable reduction in strength. Larger particle sizes also make it more difficult to obtain an even distribution of the treated lime in a cement mixture. The treated lime qualities preferably have a particle size of 1 - 100 µm, preferably 5 - 60 µm.

The starting material used according to the invention comprises at least 36% CaO, with the remainder preferably essentially consisting of Ca(OH)2 and/or CaCO3. The expression "consisting essentially of" is used to imply that the starting material may contain smaller amounts, as a rule below 10%, of other constituents which do not prevent the formation of an effective, protective coating, e.g. such impurities as silicates and oxides of iron, magnesium and aluminium. When the treated lime is produced by heating the starting material in the presence of steam and/or carbon dioxide, the starting material preferably contains at least 80%, in particular, at least 90%, CaO. Dolomitic lime qualities that contain smaller amounts of CaO

are thus preferred starting materials.

If the atmosphere includes water vapour, but no carbon dioxide, the temperature must be 100 - 400°C, preferably 150 - 400°C, and preferably 150 - 250°C. For a given CaO residual content, the higher the treatment temperature, the higher the expansion activity of the product. At temperatures below 100°C, the lime is hydrated, but no effective protective coating is formed. At temperatures above 400°C, calcium hydroxide is not formed, or if it is present initially, it is at least partially decomposed. To ensure a sufficient protective coating, heating in an atmosphere comprising water vapor should preferably be continued for such a time that the particles will gain an increased weight by at least 2%, e.g. 2 - 16%, especially 4 - 12%, and preferably 5 - 11%, based on the weight of the starting material. If the starting material essentially consists of CaO, these weight increases will lead to a product containing at most 92%, e.g. 92 - 36%,

and preferably 84 - 50%, active residual lime as a core and a protective coating which essentially consists of calcium hydroxide.

If the atmosphere includes carbon dioxide, a temperature of 350 - 850°C is necessary to convert CaO into CaC03.

At temperatures below 350°C, the reaction is between CaO and CO2

very slow, and no effective protective coating is formed. At temperatures above 850°C, CaCO^ is not formed or, if it is present initially, it will decompose. To ensure a sufficient protective coating, heating in an atmosphere comprising C02 should preferably be continued for such a time that the particles will gain an increased weight by at least 2%, e.g. 2 - 28%, especially 4 - 16%, and preferably 5 - 16%, based on the weight of the starting material. If the starting material essentially consists of CaO, these weight increases will lead to a product containing at most 95%, e.g. 36 - 95%, especially 64 - 91%, and preferably 64 - 89%, active residual lime as a core and a protective coating which essentially consists of calcium carbonate.

It is also possible to form a protective coating comprising both CaCO^ and Ca(OH)2 by heating the starting material in an atmosphere comprising CC^ and water, preferably at a temperature of 350 - 400°C. Under such conditions, the heating should preferably be continued for such a time that the particles will gain an increased weight of 4 - 14%, especially 5 - 12%, as these weight increases correspond to (for a starting material which essentially consists of CaO) end products which contains 90 - 40%, especially 80 - 50%, active residual lime.

As indicated above, grades of lime which have been partially hydrated and/or carbonized at temperatures below 100°C do not have an effective protective coating and are not useful as expansion additives. Usable expansion additives can be obtained from such partially hydrated and/or carbonated lime grades by using these as starting materials for the above-described processes which include heating in an atmosphere comprising water vapor and/or CO2, provided that the product has a sufficient calcium oxide residual content. According to the invention, however, it has been shown that partially hydrated lime grades can also be converted into usable expansion additives by heating them at temperatures of at least 100°C, preferably below 500°C, e.g. 200 - 400°C, in an atmosphere that does not contain water vapor or carbon dioxide. According to this embodiment of the invention, the starting material can comprise 8 - 64%, preferably

16 - 50%, CafOH^, the remainder preferably essentially consisting

of CaO. For a given CaO content, the higher the treatment temperature and the longer the duration of the treatment, the greater the expansion activity of the product.

Treated lime grades that already have a certain expansion activity can be further treated using one of the methods described above to make them more active. However, it should be noted that such further treatment, especially if it is carried out in an atmosphere comprising water vapor at relatively low temperatures, e.g. below 200°C, especially 150°C, can reduce the residual CaO content in the treated lime so strongly that this activity will be reduced.

The heat treatments described above can be carried out in any suitable way that does not include grinding down the particles, which would have been able to destroy the protective coating. For small quantities of the product, the starting material can simply be spread out on a pan. For larger quantities, fluidized bed methods can be used.

The new expansion additives can be incorporated into hydratable cement mixtures in any suitable way, but according to the invention it has been shown that optimal expansion activity is obtained if the additive is added to a mixture of at least part of the cement and in the least part of the water. The cement mixtures can of course contain sand, aggregates and other common additives,

but a presence of additional quicklime, hydrated lime or carbonated lime should preferably be avoided. The new additives have delayed reactivity towards water, as measured by ASTM Test C110-71, Section 9, by up to 100 minutes, but this does not account for the fact that they show a much stronger delayed reactivity in cement mixtures. It is believed that this unexpectedly delayed reactivity is due to some form of interaction between the protective coating and the constituents present in the cement slurry. The amount of the additive used will, among other things, be dependent on its CaO content and on the desired strength and expansion for the final product and will generally be 3 - 30%, preferably 3 - 10%,

and preferably 5-7%, based on the combined weight of the hydratable cement and admixture. Especially if the treated lime grades have been carefully treated, their presence will have little or no detrimental effect on such important properties of the cement mixture as water requirement, consistency, working time and tendency to "false setting".

The invention is described in more detail by means of

the examples below. The expansion activities of the various samples prepared according to Examples 1-7 are set forth in Table 1, which shows the percentage increase in length for sample prisms (ie, sample bars) which (unless otherwise indicated) were cured in water after casting in accordance with with AS TM C490-70 and C157-69T from mortar prepared in accordance

with ASTM C305-65 and containing 1 part of a mixture of the treated lime sample and a type III Portland cement ("Norcem Rapid"), 2 parts of washed Monterey sand No. 20 and 0.4 part of water. The amount a< y> the treated lime sample in the test prisms and the number of curing

days are given in brackets after each percentage figure for the expansion. The samples with a "C" after the sample number in Table 1 are comparative examples that do not conform to the invention.

Example 1

Portions of CaCO^ with a particle size below <4>4 µm were heated in a muffle furnace at 1000°C for the time necessary to prepare samples comprising CaO in an amount of 100% (sample 1), 93.6% (sample 2), 90.2% (sample 3) or 72.3%

(sample 4). Parts of sample 1 were heated in a muffle furnace at 580°C in an atmosphere of CO2 until the CaO content was 94.5%

(sample 5), 89.8% (sample 6) and 80.5% (sample 7). Portions of commercially available lime ("Flint Kote") with a particle size below 44 µm were heated in a muffle furnace at 580°C in an atmosphere of CO 2 until the CaO content was 94.8% (Sample 8), 87.5% (sample 9) or 74.8% (sample 10).

Example 2

Portions of a commercially available lime ("Flint Kote") which contained 95% CaO, the remainder being Ca(OH)2 and traces of CaCO-j, and which had a particle size of less than 44 µm, were exposed to water vapor at a temperature of 22°C (Sample 11), 60°C (Sample 12), 100°C (Sample 13), 150°C (Sample 14) or 200°C (Sample 15) until the CaO content had been lowered to level which is reproduced in table 1.

Example 3

A treated lime with expansion activity and containing 60% CaO, 27% Ca(OH)2 and 13% CaCO3 and with a particle size below lOO^um (sample 16) was exposed to water vapor at a temperature of 22°C' (sample 17), 60°C (sample 18), 100°C (sample 19), 150°C (sample 20) and 200°C (sample 21) until the CaO content was 31%

(sample 17) or 40% (samples 18 - 21).

Example 4

A treated lime with expansion activity and containing 85% CaO, 5% Ca(OH)2 and 10% CaCO3 and with a particle size below lOO^um was exposed to steam at a temperature of 110°C until the CaO content had decreased to 6 4% (sample 22).

Portions of sample 22 were heated at 110°C for 180 minutes

(sample 23), at 200°C for 15 minutes (sample 24), at 200°C for

60 minutes (sample 25), at 300°C for 15 minutes (sample 26),

at 340°C for 30 minutes (sample 27), at 340°C for 50 minutes

(sample 28), at 400°C for 90 minutes (sample 29) and at 455°C for

15 minutes (trial 30). Samples 29 and 30 lost respectively. 2% and 3%

in weight during the heat treatment due to calcium hydroxide being decomposed. The same treated lime was exposed to steam at a temperature of 200°C until the CaO content had dropped to 64%

(sample 31). A commercially available lime ("Diamond Springs") which

essentially consisting of CaO and having a particle size below 100 µm, was exposed to water vapor at a temperature of 25°C

until the CaO content was 64% (sample 32). A portion of sample 32

was heated at 340°C for 60 minutes (sample 33). Another sample of the same commercially available lime was exposed to steam at a temperature of 200°C until the CaO content had decreased to 58% (sample 34).

Example 5

Portions of a commercially available lime ("Flintkote") as used in Example 2 were exposed to steam

at a temperature of 26.0°C until the CaO content was 85% (sample 35) or 58% (sample 36). These samples were processed into mortars as described above, but with the exception that a cement of type 1-2 ("Kaiser Permanente") was used and that the amount of water was 0.36 parts. A control mortar that did not contain added lime,

was also produced. Prisms that were cast from these mortars,

was cured both in water and in air at 50% relative humidity.

Example 6

A treated lime with expansion activity and containing 84.5% CaO, 6.2% Ca(OH)2 and 9.3% CaCO3 and with a particle size of less than 100 µm was used to prepare mortars containing 0.9 part cement of type 1-2 ("Kaiser Permanente"), 0.1 part of the treated lime, 2 parts of washed Monterey sand No. 20 and 0.36 part of water. The mortars were produced using three different methods.

Method 1 (ASTM C305-65) - A. The treated lime

and the cement was dry mixed. B. The water was added and mixed in

30 seconds at low speed. C. The sand was added and mixed at low speed for another 30 seconds. D. The mixture was increased to medium speed for 30 seconds. E. Pause for 90 seconds. F. Mixing was done at medium speed for 1 minute.

Method 2 - A. The cement and water were mixed

for 30 seconds. B. The treated lime was added and the mixing continued for 30 seconds. C-F. Same as for method 1.

Method 3 - A. The treated lime and water were mixed for 30 seconds, after which the cement was added and mixed for 30 seconds. D-F. Same as for method 1.

The expansions for prisms that were cast from these mortars are reproduced in table 1 under samples 37, 38 and 39 (respectively methods 1, 2 and 3).

Example 7

A commercially available lime ("Flintkote") was divided into a first fraction in which the particles had a size of 45 - 425 µm, and in a second fraction in which the particles had a size of less than 45 µm. Both fractions were heated for 1 hour at 950°C and then treated with steam at 200°C until the CaO content was 69.5% (first fraction, sample 40) or 73%

(second fraction, sample 41).

Example 8

A commercially available lime ("Cementa") was treated with steam at a temperature of 250°C until the CaO content was 80%, and it was then divided into a first fraction in which the particles had a size below 74 µm, and in another fraction in which the particles had a size of 75 - 125 yum. The first fraction was used to prepare mortars containing 0.954 part of Type 1 cement ("Norcem PC 300"), 0.046 part of the treated lime, 3 parts of washed Monterey sand No. 20 and 0.5 part of water. The second fraction was used to prepare mortars in the same way, but with the difference that .0.96 part of the cement and .04 part of the treated lime were used. The compressive strength of prisms cast from these mortars was measured after curing in water for the indicated number of days, and is reproduced in Table 2.

Example 9

Portions of a commercially available lime ("Flintkote"), as used in Example 2, were treated with steam at a temperature of 200°C until the CaO content was 91%

(nrøve 42), 70% (prtve 43) or 49.5% (ortve 44) or with C07 at 580°C until the CaO content was 88% (ortfve 45) or 68%

(sample 42), 70% (sample 43) or 49.5% (sample 44) or 68%

(sample 46). A part of sample 45 was treated with steam at a temperature of 200°C until the CaO content was 54% (sample 47). Cement pastes were prepared by dry mixing 0.1 part of the sample and 0.9 part of a type 1-2 ("Kaiser Permanente") cement, and water (0.36 part) was then added and the mixture mixed in a mixer of type Hobart for 20 seconds at low speed followed by 20 seconds at medium speed. The pastes were filled into a Halliburton consistency measuring apparatus and the consistencies of the pastes, expressed in Halliburton consistency units, are given in Table 3 below (which also gives the results for a paste containing 1 part of the type 1 - 2 cement and no treated lime).

Claims (11)

1. Powdery material which is usable as an expansion additive for hydratable cement mixtures, and which comprises calcium oxide and other ingredients which delay hydration of the calcium oxide, characterized in that it essentially consists of particles with a size below 250 yum and comprising a core of calcium oxide and around the core a protective coating of calcium hydroxide and/or calcium carbonate, and that it contains 36 - 95% by weight calcium oxide and has an expansion factor of at least 0.06%. 2. Material according to claim 1, characterized in that its calcium oxide content is 50 - 84% by weight, and that the protective coating essentially consists of calcium hydroxide. 3. Material according to claim 1, characterized in that its calcium oxide content is 64-91% by weight, and that the protective coating essentially consists of calcium carbonate. 4. Material according to claims 1-3, characterized in that the particles have a size of 1 - 100 µm. 5. Material according to claims 1-4, characterized in that it has an expansion factor of 0.07 - 0.4%. 6. Method for the production of a material according to claims 1-5, characterized in that particles which (a) have a size of less than 250 µm and (b) comprise at least 36% by weight CaO, are heated, the particles being kept at a temperature of at least 1V00°C and, if the particles originally contain less than 8% by weight of Ca(OH)2, the particles are kept at a temperature of 100 - 400°C in an atmosphere comprising water vapour, or at a temperature of 350 - 850°C in an atmosphere which includes carbon dioxide, as the heating is carried out under such conditions that the product will contain 36 - 95% by weight of CaO and have an expansion factor of at least 0.06%. 7. Method according to claim 6, characterized in that particles are used that originally comprise at least 80% by weight CaO and that they are heated at a temperature of 100 - 400°C in an atmosphere that includes water vapor for a sufficient time to increase the weight of the particles by 4 - 12%. 8. Method according to claim 6, characterized in that particles are used which originally comprise at least 80% by weight of CaO, and that the particles are heated at a temperature of 350 - 850°C in an atmosphere comprising C02 for a sufficient time to increase the weight of the particles by 4 - 16%. 9. Method according to claim 6, characterized in that particles are used which originally comprise at least 80% by weight of CaO, and that the particles are heated in an atmosphere comprising C02 and water vapor for a sufficient time to increase the weight of the particles by 4 - 14%. 10. Method according to claim 6, characterized in that particles are used which originally comprise 16 - 50% by weight Ca(OH)2, and that the particles are heated at a temperature of 100 - 400°C. 11. Use of the powdery material according to claims 1-5 in a cement mixture comprising a Portland cement, water and the powdery material in an amount of less than 30%, preferably 3-10%, based on the total weight of the cement and the powdery material.